Recently, electric/electronic appliances having compact sizes with light weight, such as cellular phones, notebook computers and camcorders, have been actively developed and produced. Such electric/electronic appliances are equipped with battery packs so that users can use the electric/electronic appliances in various places even if electric power sources are not separately provided for the electric/electronic appliances. A battery pack includes at least one battery capable of outputting an operational voltage at a predetermined level in order to operate the electric/electronic appliances for a predetermined period of time.
Secondary batteries, which are rechargeable batteries fabricated in small sizes with high capacity, are currently employed in the secondary battery packs due to their economical advantages. Secondary batteries are extensively used as power sources for portable electronic appliances, such as camcorders, portable computers, and portable phones. For instance, Ni-MH batteries, Li-ion batteries and Li-ion polymer batteries have been developed as power sources for the portable electronic appliances.
In particular, lithium secondary batteries have an operational voltage of more than 3.6V, which is three times higher than that of Ni—Cd batteries or Ni-MH batteries used as power sources for the portable electronic appliances. In addition, lithium ion secondary batteries have high energy density per unit weight, so lithium ion secondary batteries are extensively used in advanced electronic technology fields.
A Li-ion secondary battery includes a bear cell, which is fabricated by accommodating an electrode assembly including a positive electrode plate, a negative electrode plate and a separator in a can made from aluminum or an aluminum alloy together with an electrolyte and sealing the can with a cap assembly.
In a typical polymer secondary battery, in which an electrode plate or a separator is made from polymer, the separator acts as the electrolyte or is impregnated with an electrolyte component, so the electrolyte is not leaked or leakage of the electrolyte is significantly reduced. Thus, a pouch can be used instead of the can.
In a typical Li-ion secondary battery, an electrode plate is formed by coating slurry (also referred to as a material forming an electrode coating portion) including electrode active materials (lithium oxides for a positive electrode (cathode) and carbon materials for a negative electrode (anode)) onto an electrode collector made from metal foil.
The slurry can be formed by mixing a solvent with a plasticizer, an electrode active material and a binder. In addition, a negative electrode collector is mainly made from copper and a positive electrode collector is mainly made from aluminum. The binder includes PVDF (Poly Vinylidene Fluoride) or SBR (Styrene Butadiene Rubber), and the solvent includes acetone or NMP (N-Methyl Pyrrolidone). It is also possible to use water as the solvent.
The electrode assembly includes a positive electrode plate, a separator, and a negative electrode plate, which are strips sequentially stacked and rolled in the form of a jelly roll, or spiral.
A slit die is formed on at least one surface of the electrode collector forming the positive electrode plate or the negative electrode plate. Slurry is fed into the slit die formed on the surface of the electrode collector, so that an electrode coating portion is formed on the surface of the electrode collector.
The slurry fed into the slit die of the electrode collector is a fluid including a solvent and a binder. The solvent is volatilized through a dry process so that the slurry is bonded to the electrode collector with sufficient bonding strength by means of the binder.
The electrode active material is coated on the electrode collector corresponding to a length of an electrode while forming an uncoated portion between the electrode coating portions in order to allow an electrode tab to be welded to the uncoated portion. Accordingly, the electrode collector includes the electrode coating portions and the uncoated portion.
However, although it may vary depending on slurry coating apparatuses, the slurry may be conglomerated on a coating start portion and a coating end portion of the electrode collector, so the coating start portion and the coating end portion of the electrode collector may slightly protrude as compared with other coating portions. Such protruding portions are formed at both ends of the electrode coating portions of the positive electrode plate and the negative electrode plate.
For this reason, pressure is concentrated on the protruding sections when winding the electrode assembly or external impact is applied to the electrode assembly, so the separator used for insulating the positive electrode plate from the negative electrode plate may be damaged. If a short circuit is generated between the positive electrode plate and the negative electrode plate due to the damage of the separator, not only is a yield rate of the secondary batteries reduced, but also an accident may occur.
In order to solve the above problems, an insulation layer is conventionally formed on the protruding sections of the electrode coating portions coated on at least one surface of the positive electrode plate or the negative electrode plate, thereby preventing the positive electrode plate from making contact with the negative electrode plate and preventing the separator from being damaged due to the protruding sections.
However, in this case, the insulation layer may cover a part of the electrode coating portion, so the reaction area of the electrode coating portion is reduced. Accordingly, capacity of the secondary battery is reduced proportionally to the reduction of the reaction area of the electrode coating portion.
That is, since the capacity of the secondary battery is proportional to the reaction area of the electrode coating portion of the positive electrode and the negative electrode, if the reaction area of the electrode coating portion is reduced due to the insulation layer, the capacity of the secondary battery is also reduced.